A Definitive Survey for Lyman Limit Systems at z~3.5 with the Sloan Digital Sky Survey

Understand

We perform a semi-automated survey for tau>=2 Lyman Limit systems (LLSs) in quasar spectra from the Sloan Digital Sky Survey, Data Release 7.

• From a starting sample of 2473 quasars with zem=3.6-4.4, we analyze 469 spectra meeting strict seletion criteria for a total redshift path Dz=93.8 and identify 192 intervening systems at z>3.3.
• The incidence of tau>=2 LLSs per unit redshift, l(z), is well described by a single-power law at these redshifts: l(z) = C_LLS [(1+z)/(1+z_*)]^gamma, with z_*=3.7, C_LLS = 1.9+/-0.2, and gamma = 5.2+/-1.5 (68% c.l.).
• These values are systematically lower than previous estimates (especially at z<4) but are consistent with recent measurements of the mean free path to ionizing radiation.